Internal combustion engines may include water injection systems that inject water into a plurality of locations, including an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx, while a more efficient fuel mixture may reduce carbon monoxide and hydrocarbon emissions.
The cooling effect of water injection advances combustion phasing (e.g., advances the CA50 of engine combustion). This allows fuel efficient spark timing adjustments to be made. One example of adjusting the spark timing with the injection of water is shown by Fried et al. in U.S. Pat. No. 8,434,431. Therein, spark timing is advanced while water is injected into an engine. This, in turn, shifts an engine towards a higher efficiency point.
The inventors herein have recognized potential issues with the approach of '431. As one example, the approach may cause the spark timing to bounce around. For example, prior to water injection, the engine may be operating with spark retarded by a significant amount. Then, the spark timing may be advanced quickly responsive to the water injection. However, due to various engine operating conditions, other than the water injection, that affect the torque, such as the octane rating of the injected fuel, EGR flow rate, manifold humidity, etc., the final engine operating point with the advanced spark and the water injection may not be an optimum one. Consequently, to avoid knock, spark timing may have to be retarded again. The frequent and rapid advancing and retarding of spark timing can result in an unstable torque delivery, increased NVH (noise, vibration, and harshness), and decreased fuel economy. As a result, the full potential of the water injection is not realized.
The inventors herein have recognized that the effect of the water injection on the torque ratio is not the same at all spark conditions. By continuously monitoring the torque ratio, and calibrating the water injection as a function of the torque ratio, spark timing adjustments and water injection adjustments may be better coordinated. In particular, the engine may be operated at the most efficient point while using spark and water optimally. In one example, this is achieved by a method for an engine comprising: adjusting an amount of water injection into an engine responsive to a torque ratio at a current spark timing relative to torque ratio at borderline knock, and further based on sensed humidity in an engine intake manifold.
As an example, when water injection conditions are met, a torque ratio of the current engine operating point (herein also referred to as the current torque ratio) may be determined based on the current spark timing. This torque ratio is then compared to the torque ratio of engine operation at borderline spark (BDL) (herein also referred to as the borderline torque ratio). This includes comparing a magnitude of the difference. In addition, a rate of change of the torque ratio (or torque ratio profile) may be determined for the given engine load condition. If the current torque ratio is within a threshold distance of the borderline torque ratio (such as may occur when spark retard is between 0-5 CAD of MBT), the injection of water may have a minimal effect. During such conditions, the controller may opt to conserve water and rely only on spark usage for torque control. If the torque ratio is more than a threshold distance away from the borderline torque ratio, and further if the torque ratio profile indicates a rapid rate of change (such as may occur when spark retard is between 20-30 CAD of MBT), then the controller may infer that water injection can significantly improve the torque ratio. During such conditions, the controller may opt to inject water at a maximum possible rate while advancing spark timing. For example, water may be injected via a manifold injector until the intake humidity of the manifold reaches a saturation limit. As a result of the water injection, the torque ratio may be moved to a higher efficiency point. Water injection may be reduced after the torque ratio has remained at the higher efficiency point for longer than a threshold duration.
In this way, water injection may be better coordinated with spark usage to leverage the cooling effect of the water injection. The technical effect of using a monitored torque ratio to assess the efficiency improvement of a water injection is that spark timing may be better placed, and the bouncing of spark timing may be reduced. By using water injection based on the real-time effect of the injection on the torque ratio of an engine relative to the torque ratio at BDL, water may be used more judiciously. By limiting water injection to conditions when the engine efficiency improvement is significant, water may be conserved for conditions when it is needed more. As a result, the benefits of water injection may be extended over a longer duration of a drive cycle. By adjusting water usage based on the torque ratio while operating the engine with one or more cylinders deactivated, engine operation in a VDE mode may be extended, improving fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.